Chromium plating process



United States Patent 3,442,777 CHROMIUM PLATING PROCESS Roger M. Woods, Washington, D.C., and David R. Moul, Seat Pleasant, Md., assignors to Corillium Corporation, Arlington, Va., a corporation of the District of Columbia No Drawing. Filed June 20, 1966, Ser. No. 558,568 Int. Cl. C23b 5/06, 5/72 US. Cl. 204-51 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process of chromium plating and more particularly to a process for producing a lustrous uniform golden or golden bronze chromium finish on a wide variety of conductive base materials.

An object of this invention is to provide a process of electrodepositing an extremely adherent corrosion resistant golden film upon a conductive base which film exhibits excellent wear resistance and which protects the base metal from corrosion. Another object of the invention is to provide a process using conventional chromium plating equipment wherein certain chemical compounds and conditions are provided for obtaining a golden chromium plate. Another object of the invention is to provide a process for finishing decorative trim components for use in the automotive and appliance fields wherein a golden chromium plate is obtained which has an attractive and unique decorative appearance and also has excellent wearing properties. Other objects and advantages of the invention will become readily apparent from the following detailed description of the invention.

According to the invention, a golden chromium plate is obtained on a conductive base material in a system in which the base metal is cathodic and in which the electrolyte comprises an aqueous solution of chromic anhydrides (CrO containing phosphoric acid and sulfuric acid in the solution. The solution contains from about 15 grams to 75 grams per liter of chromic anhydride and from about .5 gram to about 50 grams per liter of phosphoric acid (H PO 85%), and from about .10 gram to about .30 gram per liter of sulfuric acid (H 80 The sulfuric acid is preferred, although salts of sulfuric acid can be used, such as sodium sulfate. It is necessary to stay within the above stated ranges in order to produce a suitable golden chromium plate.

It is common practice in the conventional procedures for producing chromium plate to employ sulfuric acid in the plating bath with chromic anhydride. However, those procedures normally use chromic acid concentrations of about 250 grams per liter and higher. When sulfuric acid is employed in the bath with the chromic anhydride, the ratio between the chromic anhydride and the sulfuric acid is in the range of about 50/1 to about 150/1 and normally about 100/1. In this invention, the concentration of chromic anhydride is used in lower than the normal concentrations used and the addition of the amounts of phosphoric acid and sulfuric acid to the bath in the stated amounts, gives the golden color to the chromium plate produced. It was found that the golden color was not 3,442,777 Patented May 6, 1969 produced when the phosphoric acid and sulfuric acid are added to the higher chromic anhydride concentrations as employed in conventional procedures.

The object to be plated is prepared for plating by conventional plating procedures, which may include degreasing, alkaline cleaning, acid dipping, polishing, sandblasting, etc.

The preferred electroplating bath comprises an aqueous solution containing 30 to 55 grams per liter of CrO and 10 grams to 36 grams per liter of phosphoric acid, and .15 to .28 gram per liter of sulfuric acid.

The plating bath is prepared in the concentrations as stated above and added to a plating tank to the desired operating level. The tank must be thoroughly cleaned to prevent bath contamination, particularly where chloride or fluoride type baths have previously been used. Anodes are then placed in the tank and electrolyzed with dummy cathodes for about 1 minute. The solution is frequently stirred during this period as by air agitation. The bath is then ready for use. The object or part to be plated is then immersed in the bath and is made the cathode of the electroplating system. Anodes of various materials may be employed, but it has been found preferable to use steel anodes over the carbon or lead anodes. Lead and carbon anodes may be employed, but with less satisfactory results. The tank which holds the electrolyte solution is preferably lined with a synthetic resin material such as polyvinyl chloride or other suitable inert material. The plating tank should also be provided with an exhaust system to carry away fumes of the chromic acid.

The plating bath should be operated at a temperature of about 50-l20 F. and preferably at about 6085 F. It was found that the particular temperature used within these ranges has little effect on the color of the deposit. In the lower temperature ranges, about 50-65 F., the color darkens slightly, to a little more bronze color. In the higher ranges, about l20 F., the color lightens very slightly, to a yellow gold effect. The best deposits will usually be obtained at a cathode current density of between 50 and 600 amperes per square foot, and preferably about 100-300 amperes per square foot.

Plating times will vary according to the thickness desired and the complexity of the object to be plated, but normally the time will vary between about 30 seconds to about ten minutes for decorative applications, and longer where thicker deposits are required.

After the object has been plated, it is removed from the bath and rinsed in a conventional manner and is allowed to dry. The covering power is approximately that of conventional chromium plating, and in cases of extremely deep recesses, internal anodes may have to be used, the same as in conventional chromium baths.

It has been found desirable to maintain the anode area as large as possible with respect to the cathode area in order to maintain the anode current density at a relatively low value to avoid coating the anode with a film, the exact nature of which is not fully understood, but which tends to increase the resistance of the system. The anode area should be as great as and preferably three or four times as great as the cathode area for most efiicient operation.

The plating bath may be replenished by the addition of chromic anhydride and sulfuric acid and phosphoric acid as they are consumed to bring the concentration of the ingredients in the bath back to the concentrations of the ingredients originally present in the bath. A sufiicient accurate control for production use may be achieved by conventional analytical techniques.

The plating bath of the invention which produces a uniform gold chromium finish is applicable to a wide variety of conductive base materials such as copper, nickel, steel, stainless steel, brass and bronze. Other metals such as zinc die cast and aluminum can also be plated after a suitable underplate is applied.

The plate produced by the above-described process is lustrous and golden in color and exhibits a lustrous ap pearance when plated over a bright or polished surface. Spectral reflectance of the plated article may be reduced by rendering the exterior article matte in appearance prior to the final plating operation. Thus, different effects can be achieved by varying the surface of the base material or underplate.

It will be appreciated to those skilled in the art that the process involves a chromic acid electrolyte having a low concentration of addition agents compared to the concentration of addition agents to conventional CrO baths with the heretofore unexpected result of obtaining a lustrous golden plating.

EXAMPLES Example 1 A conventional polyvinyl chloride lined steel tank containing a carbon steel anode is used in this example and in the following examples. A zinc die cast drawer handle was prepared for plating according to conventional procedures as heretofore described. The handle was copper and nickel plated according to conventional plating procedures. After rinsing, the handle was transferred to the steel tank 'which contained the golden plating bath which comprised a solution containing 45 grams per liter of chromic anhydride and 20 grams per liter of phosphoric acid and .21 gram per liter of sulfuric acid. The current density was 200 amperes per square foot and the temperature of the bath .was 75 F. The plating time was five min utes. The handle was then removed and rinsed in cold water, then hot water, and dried. The handle has a coating of chrome which had a golden luster.

Example 2 A steel golf club shaft was prepared for plating as in the above example and then copper and nickel plated according to conventional procedures. Thereafter, it was rinse-d and transferred to a golden plating bath which contained 55 grams per liter of chromic anhydride and 15 grams per liter of phosphoric acid and .26 gram per liter of sulfuric acid. The current density was 280 amperes per square foot and the temperature of the bath was 65 F. The golf club shaft was plated for a period of eight minutes and then removed from the bath and rinsed in cold water, then hot water, and dried. The golf club shaft had a chrome deposit which had a golden luster.

What we claim is:

1. A process of producing electrodeposited golden chromium coatings comprising plating an object in-an aqueous bath comprising about 15 to about 75 grams per liter of chromic anhydride and about .5 to about 50 grams per 4 liter of phosphoric acid and about .10 to about .30 gram per liter of sulfuric acid, using a current density at the cathode of about 50 to about 600 amperes per square foot and a temperature of about 50 F. to about 120 F., for a period of time sufiicient to give a chromium coating of desired thickness.

2. The process of claim 1 wherein said bath comprises about 30 to about grams per liter of chromic anhydride and about 10 to about 36 grams per liter of phosphoric acid and about .15 to about .28 gram per liter of sulfuric acid.

3. The process of claim 1 wherein said current density is about 100 to 300 amperes per square foot.

4. The process of claim 1 wherein said temperature is about to F.

5. The process of claim 1 'wherein said period of time is about 30 seconds to about ten minutes to obtain decorative golden coatings.

6. The process of claim 1 wherein said object to be plated has a previously deposited layer of metal other than chromium.

7. The process of claim 1 wherein the object to be plated is the cathode and the anode is a steel anode.

8. The process of claim 7 wherein the anode area is at least as large as the cathode area.

'9. A process of producing electrodeposited golden chromium coatings comprising plating an object in an aqueous bath comprising about 15-75 grams per liter of chromic anhydride and about .5 to about 50 grams per liter of phosphoric acid and an amount of sodium sulfate equivalent to about .10 to about .30 gram per liter of sulfuric acid, using a current density at the cathode of about 50 to about 600 amperes per square foot and a temperature of about 50 F. to about F., for a period of time sufiicient to give a chromium coating of desired thickness.

References Cited UNITED STATES PATENTS 526,114 9/1894 Placet et a1 20451 1,985,308 12/1934 Bornhauser 20451 2,042,611 6/1936 Lukens 20451 OTHER REFERENCES Griffin, John L.: Experimental Chromium Electrodeposition as Elfected by a Variety of Possible Catalysts, Plating, vol. 53, No. 2, pp. 196-203, February 1966.

ROBERT K. MIHALER, Primary Examiner.

G. L. KAPLAN, Assistant Examiner.

US. Cl. X.R. 204-37 

